DNA sequencing gel medium is a thin and fragile gel of 5 to 8% polyacrylamide, usually 0.4 mm in thickness (range 0.2 mm to 0.6 mm), and containing 7-8 M urea, and sometimes sucrose as solute, in addition to buffer electrolytes. If 35-S is used as the radionuclide label, the radioactvitity cannot penetrate either the thickness of the gel, dried urea in the gel, or even a thickness of plastic wrap. Therefore the gel must be washed free of urea and dried to a very thin layer lacking any water, and then exposed in direct contact with the X-Ray film. If 32-P is used as the radionuclide label, the gel may be autoradiographed while still wet and covering with plastic wrap, but increased resolution results from drying.
After the gel is run, the manipulations of soaking, fixing, and removal of the gel from one the glass gel plates, and placing it on a sheet of filter paper to dry down under vacuum, is a series of maneuvers requiring great skill and practice to avoid wrinkling, tearing or destroying the gel. Even for a practiced professional, this is a step of high psychological stress, since it comes near the end of the DNA sequencing procedure, and at the end of the most tedious, labor-intensive step of DNA sequencing, and the gel potentially contains data for many hundreds of nucleotides of DNA sequence.